VIRGINIA BEACH, Va.  Wildlife officials are trying to determine what is killing hundreds of sea birds that have washed ashore in Virginia Beach and other locations along the Atlantic coast in the past several weeks.

Most of the birds are greater shearwaters, which are now migrating north from their breeding grounds in the South Atlantic.

Since June 12, more than 500 dead sea birds have been reported from Maryland to Florida, said Emi Saito, a wildlife disease specialist with the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis.

"It's unusual to see so many," Saito said.

Wildlife pathologists are examining the birds for exposure to toxins, pollutants and infections, she said.

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Staffers at Back Bay National Wildlife Refuge in Virginia Beach recently found about a dozen dead greater shearwaters on the beach, said Dorie Stolley, a federal wildlife biologist at the refuge.

Similar reports have come from the Outer Banks of North Carolina, as well as Myrtle Beach and Hilton Head in South Carolina.

Almost 200 birds have washed up in South Carolina, said Diane Duncan, an ecologist with the U.S. Fish and Wildlife Service in Charleston, S.C. "In 20 years here, I have never seen this kind of mortality event," Duncan said. "It certainly is a concern to us, and we'd like to know the cause."

Tests on two of the birds ruled out toxins found in red tide, a type of algal bloom that biologists initially suspected as a culprit, Duncan said.

Will Post, an ornithologist and curator at The Charleston Museum, said he dissected six greater shearwaters that had washed up alive, unable to fly, and later died. The birds' stomachs were empty, but they had varying levels of fat reserves, suggesting they did not die of starvation, he said.

The greater shearwaters are common birds that resemble gulls in appearance and size, with brown to gray heads and white undersides, webbed feet and dark, tube-like bills.

They typically stay far offshore, where they feed on small fish and squid.

Shearwaters fly nearly 5,000 miles during their annual migrations to and from nesting grounds on Tristan da Cunha, a chain of volcanic islands in the South Atlantic, Post said. The cold-water birds breed in April and May and then fly to their summer grounds off New England and points north, he said.